Birefringence and Dichroism under Shear
The setup allows the time resolved measurement of dichroism and birefrigence of a sample sheared in a Couette cell. Therefore, the reaction of the sample on an accelerated or deaccellerated shear flow can be analysed on the time scale of milliseconds. The temperature of the Couette cell can be controlled in a moderate range close to room temperature.
HeNe Laser
P1 Lens Couette P2
cell with bath
Bs1 Bs2
D2
D1D3 Lockin
Amplifier
ω λ−2 Babinet
λ−4
Figure1: Schematic of the bifrigence and dichroism set-up
Figure2: Set-up on specially designed optical bench
The main part of the set-up is a λ/2 retardation plate which rotates with the frequency ω (about 4kHz) on the hollow axis of a dentist drill.
Together with the Glan- Thompson-polarizer P1, this produces at the sample a linear polarized laser beam rotating with the frequency 2ω. (The lens is needed to focus the beam through the narrow hollow axis of the dentist drill.) The transmitted intensity signal of diode D1 is fed together with the rotational frequency of the dentist drill to a lock-in amplifier in order to extract the 4ω frequency component of the D1-signal which determines the dichroism. The same treatment of the signal of diode D2 after passing a λ/4 plate and the second Glan-Thompson polarizer P2 in a configuration as an analyser for circular polarized light yields the additional birefringence contribution. Any offset-value and angle of the birefringence that might be due to the windows of the Couette cell or the temperature bath can be removed with a Soleil-Babinet compensator. D3 is used (when mounted) for turbidity measurements.
The optical bench for this set-up was specially designed so that the whole set-up can brought to a vertical position with respect to the Couette cell. In this configuration the polarization properties of the sample are probed in the vorticity direction instead of dichroism and birefrigence along the gradient direction.
Further information: H.Kriegs, P. Lettinga
